Real estate investment trusts that buy mortgage debt tumbled, adding to their biggest weekly loss in more than a year, on concern the markets that finance them will be roiled if the U.S. government defaults on its debt. A Bloomberg index of shares of 32 mortgage REITs, including New York-based Annaly Capital Management Inc. (NLY) and Atlanta-based Invesco Mortgage Capital Inc. (IVR), dropped as much as 8.5 percent today, the most since May 6, 2010. The stocks pared the losses to 2 percent at 4:15 p.m. in New York.